(Prior Art)
Polyesters such as polyethylene terephthalate are excellent in moldability and biaxially-stretchable and give structures which excel in clarity, mechanical strength, heat resistance and impact resistance and are moderately good in gas barrier property. Therefore, such structures have found broad application as containers for foods such as carbonated beverages. However, being moderately good, the gas barrier property of polyester structures is not as good as desired, so that foods in polyester containers cannot keep long. Improvements, therefore, have been awaited.
For the purpose of improving the gas barrier property of polyester articles, attempts have been made to provide a multi-layer structure consisting of at least one polyester layer and at least one gas-impermeable ethylene-vinyl alcohol copolymer layer by utilizing the multi-layer extrusion, extrusion coating, or other technique but as the mutual adhesive affinity of the polyester and ethylene-vinyl alcohol copolymer is low, improvements in this interlayer adhesive bond strength are desired.
For the purpose of improving the above-mentioned interlayer bond strength, there has recently been proposed a method in which a defined thermoplastic polyester, i.e. a polyester consisting of terephthalic acid and isophthalic acid in the proportions of 95 to 40 mol percent and 5 to 60 mol percent, respectively, based on the total dibasic acid component, is interposed as an adhesive layer between the above-mentioned two heterogenous layers and the two layers are intimately welded together in the molten state of the adhesive (Japanese Patent Application Kokai 59-143637) or a method in which a thermoplastic polyester comprising terephthalic acid in a proportion of at least 40 mol percent of the total dibasic acid component and containing ether bonds in a proportion of 17 millimols per 100 grams of the resin within its backbone chain (Japanese Patent Application Kokai 59-143638) is interposed as an adhesive layer between said two heterogenous layers and the two layers are then intimately welded together in the molten state of the adhesive.